Method of controlling the movements of the squeeze plates of a string moulding apparatus and string moulding apparatus

ABSTRACT

The present invention relates to a method of controlling the movements of the squeeze plates of a string moulding apparatus, and to an apparatus for carrying out the method. Such an apparatus generally comprises a moulding chamber defined between two squeeze plates. One of the squeeze plates can be pivoted to open the moulding chamber. The production cycle of an apparatus of said kind comprises apart from the charging of the chamber with mould material several movements of the squeeze plates. A movement of one plate commences before the preceding movement of the other plate has finished. The hydraulic system of the apparatus comprises two pumps, one pump for each actuator associated to one of the squeeze plates.

TECHNICAL FIELD

The present invention relates to a method of controlling the movementsof the squeeze plates of a string moulding apparatus and to an apparatusfor carrying out the method. Such an apparatus comprises generally amoulding chamber defined between two squeeze plates. One of the squeezeplates can be pivoted to open the moulding chamber.

BACKGROUND ART

A method of this general kind is known from U.S. Pat. No. 5,647,424.According to this method, the squeeze plates carry out a number ofsequential movements in order to produce a mould. The moulding processcomprises the steps of charging the moulding chamber with compressiblemould material, e.g. clay-bonded green sand, pressing the mould materialbetween a squeeze plate and a pivoted squeeze plate thus forming themould, retracting the pivoted squeeze plate and pivoting the pivotedsqueeze plate out of the way, moving the squeeze plate towards and pastthe pivoted squeeze plate for pushing the mould out from the mouldingchamber and bringing it into abutment with a mould having been producedimmediately before, and moving the squeeze plates back to theirrespective starting positions, whereafter a new cycle begins.

The forces exercised during the mould squeezing are of considerabledimension. Moreover, in order to produce high quality moulds it isnecessary to provide exact guiding for the squeeze plates which canwithstand bending forces that are caused by the reactive forces of themould material not always being distributed evenly across the frontsurface of the squeeze plates with their associated patterns so that theresultant of these forces is not parallel to the axis of the mouldingchamber. Thus, the actuators and the associated guiding system tend tobe heavy constructions that can both withstand these forces and providethe required precise guiding. Consequently, the speed with which thesqueeze plates can move is relatively low due to the large inertia ofthe elements to be moved. Attempts to reduce the length of the operatingcycle of these types of machines by increasing the speed of themovements of the squeeze plates have consequently not been verysuccessful.

DISCLOSURE OF THE INVENTION

It is the object of the present invention to provide a method ofcontrolling the movements of the squeeze plates of a string mouldingapparatus of the kind referred to above which allows a shorter operatingcycle without increasing the speed of the movements, thus resulting in ahigher production. This object is achieved with a method of controllingthe movements of the squeeze plates of a string moulding apparatus ofsaid kind as discussed in detail hereinafter. With this method, themovement of one of the squeeze plates can commence before the movementof the other squeeze plate has finished and thus the production rate canbe increased.

According to an embodiment of the invention, the movement of the squeezeplate further into and past the moulding chamber and past the pivotedsqueeze plate to transport the mould beyond the pivoted squeeze platestarts at such a time that the mould face formed by the pivoted squeezeplate will reach the chamber front just after the moment, where thepivoted squeeze plate starts its pivoting movement. In order to achievethis timing, the distance between mould surface of the squeeze plate tothe moulding chamber front is taken into account.

According to a further embodiment of the invention, the pivotingmovement of the pivoted squeeze plate back into the moulding chamber toresume its starting position is started when collision between thepivoted squeeze plate and the retracting squeeze plate is excluded.Hereto the thickness of the pattern associated with the squeeze plate isalso taken into account.

It is a further object of the present invention to provide a stringmoulding apparatus of the kind referred to above for carrying out themethod. This object is achieved with a string moulding apparatus of saidkind as discussed in detail hereinafter. With this apparatus, themovement of one of the squeeze plates can commence before the movementof the other squeeze plate has finished and thus the apparatus has ahigher production rate.

According to yet another embodiment of the invention, the pumps arevariable displacement pumps. This embodiment does not require the use ofproportional valves, thereby reducing the amount of throttling of thehydraulic fluid.

According to a further embodiment of the invention, the pumps are fixeddisplacement pumps. In order to do without proportional valves, thepumps are driven at a variable speed.

According to a further embodiment of the invention the pumps aredouble-sided pumps. This embodiment allows braking energy to be returnedto the pump.

According to a further embodiment of the invention, the first hydrauliclinear actuator is connectable in a closed circuit with the onedouble-sided pump and the second linear hydraulic actuator isconnectable in a closed circuit with the other double-sided pump. Withthis embodiment, the system can be operated with a certain amount ofpre-tension resulting in a better positional control.

According to a further embodiment of the invention, the first and secondhydraulic linear actuators are connectable in an open circuit to thefirst and second pumps, whereby the delivery conduit of the firsthydraulic linear actuator is connectable to the delivery conduit of thesecond linear hydraulic actuator so that the hydraulic pressure actingon the actuators is equalised. This embodiment allows the force appliedby the hydraulic actuators on the mould during compression to beequalised.

According to a further embodiment of the invention, the first and secondpumps are coupled to a common drive shaft, so that the braking energy ofone actuator can be used to drive the other actuator. With thisembodiment, the braking energy of one actuator can be transferred to theother actuator.

According to a further embodiment of the invention, the further pumps ofthe apparatus, such as servo pumps, are connected to common drive shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed part of the description, the invention will beexplained in more detail with reference to the exemplary embodiments ofthe method of controlling the movements of the squeeze plates of astring moulding apparatus and a string moulding apparatus for carryingout the method, according to the invention shown in the drawings, inwhich

FIGS. 1, 1 a, 1 b, 1 c, 1 d and 1 e diagrammatically illustrate sixstages during the production of a mould,

FIG. 2 shows a diagrammatic view of the guiding and actuating system ofthe apparatus,

FIG. 3 shows a circuit diagram of the hydraulic system for theapparatus, and

FIG. 4 shows is a plot of the speed of the squeeze plates versus time,i.e. a speed profile, according to an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIGS. 1, 1 a to 1 e, the six stages of the cycle of producing a mouldin a string moulding apparatus are illustrated. In FIG. 1, a mouldingchamber 1 is shown, of which one end is closed by a squeeze plate 2carrying a pattern in its starting position, the other end being closedby a pivoted squeeze plate 3 carrying a pattern, in this Figure shown inits lowermost (starting) position. The moulding chamber 1 is filled withcompressible mould material from a hopper 4. To the right side in thisFigure are shown two previously produced moulds 5, resting and beingconveyed stepwise on a conveyor 6, the top of which is aligned with thebottom of the moulding chamber 1.

FIG. 1a illustrates the bilateral pressing of a mould 5 in the mouldingchamber by movement of the squeeze plate 2 into the moulding chamber 1and movement of the pivoted squeeze plate 1 from the opposite side, viz.the chamber front 1 a, into the moulding chamber 1 under influence ofequally large and oppositely directed pressing forces, in this Figurebeing symbolised by arrows.

FIG. 1b illustrates the situation, in which the pivoted squeeze plate 3has been withdrawn from the moulding chamber 1 and pivoted upwardly inthe direction shown by an arrow to a position, in which all of it ispositioned at a level higher than the upper limiting level of themoulding chamber 1, thus allowing free passage below for the freshlypressed mould 5.

FIG. 1c illustrates the situation in which the mould 5 is being pushedout of the moulding chamber 1 by the squeeze plate 2 into abutment withthe last of the previously produced moulds 5 and, according to apreferred embodiment, further until it occupies the position previouslyoccupied by said previously produced mould, pushing the string of mouldsgenerally designated with 7 one step towards the right in the Figureover a distance equal to the width of a mould 5 as measured in thelongitudinal direction of the mould string 7. According to anotherembodiment, the squeeze plate 2 retracts when the mould 5 comes intoabutment with the last of the previously produced moulds. The mouldstring is then transported by a mould-string-transporting means 8.

FIG. 1d illustrates the situation in which the squeeze plate 2 is movedback to its position as shown in FIG. 1 thereby stripping the squeezeplate 2 and an associated pattern from the mould 5.

FIG. 1e illustrates the situation in which moulding chamber is closed bythe pivoted squeeze plate 3 having returned to the moulding chamber 1.Thus, both the squeeze plate 2 and the pivoted squeeze plate 3 havereturned to their starting position. The two squeeze plates 2,3automatically centre relatively to the sand injection slot, taking intoaccount the height of the pattern plates carried by them. Consequently,wear caused to the pattern plates is reduced to a minimum, and themoulding chamber 1 can be homogeneously filled. The moulding chamber ischarged again so that a new cycle may begin. During charging, thesimultaneous movement of the squeeze plates towards one another maybegin.

Between the moulds 5 casting cavities are formed, of which one is in theprocess of being cast With metal, whereas the two cavities to theextreme right in the Figures have already been cast with metal. Duringthe further movement of the string of moulds 7, the metal in the castingcavities solidifies and finally, the moulds 5 with the solidifiedcastings end up on a shake-out grate (not shown), on which the mouldmaterial is separated from the castings. Many moulds require the use ofa core (not shown) which is inserted into the moulding chamber 1 beforethe sand shot by an automatic core setter (not shown). The insertion ofthe core, after the squeeze plate 2 has returned to its startingposition, but preferably before the pivoted squeeze plate 3 has reachedits starting position, may, as in the prior art techniques, increase thecycle time.

FIG. 2 illustrates diagrammatically the construction of the stringmoulding apparatus. The movement of the pressure plate 2 is derived froma linear hydraulic actuator 10 comprising a cylinder member 11, to whichthe squeeze plate 2 is directly secured, and a piston member comprisinga piston head 12 and a piston rod 13 that passes tightly through aninner end wall 14 of the cylinder 11 and is supported by a stationaryblock 15. The stationary block 15 is an integral part of the base frameof the apparatus. The piston member divides the cylinder chamber into anouter annular compartment 16 and an inner annular compartment 17. Thepiston rod 13 is hollow and defines an inner annular chamber. A secondpiston rod 13 a extends from the outer end wall 18 of the cylinder 11into the outer annular chamber 16. A second piston head 12 a secured tothe free end of the second piston rod 13 a fits tightly in the annularchamber, thereby defining a compartment 16 a. The compartments 16, 16 aand 17 are connected to conduits 20, 21 and 22 for supply and dischargeof pressure fluid. The cylinder member 11 actually constitutes themovable element.

The pivoted pressure plate 3 comprises an analogous linear hydraulicactuator 10′ with a cylinder member 11′, a piston head 12′, a hollowpiston rod 13′, also supported by the block 15, an inner end wall 14′,an outer compartment 16′, an inner annular compartment 17′, a secondpiston rod 13 a′, an outer end wall 18′, a second piston head 12′, acompartment 16 a′and conduits 20, 23 and 24.

Also in this case, it is actually the cylinder member 11′ thatconstitutes the movable element and this cylinder member 11′ isconnected to the pivoted pressure plate 3 through a bracket 25 securedto the cylinder 11′ at the inner end thereof, said bracket 25 beingconnected through push and pull rods 26 with a frame 27 supporting thepivoted squeeze plate 3 in a hinge 28. The pivoting movement about thehinge pivoted squeeze plate 3 is caused by a lever device (not shown)forcing the pivoted squeeze plate 3 to pivot upwardly when the frame 27is moving away from the moulding chamber 1 and vice versa. When movingaway from the moulding chamber 1, the pivoting movement does not startbefore the pivoted squeeze plate 3 has reached a minimum distance thatequals at least the height of its associated pattern from the mouldingchamber.

As shown in FIG. 3, the hydraulic system of the mould string apparatuscomprises a first and second variable displacement hydraulic pumps 30and 31. The pumps 30,31 are double-sided, i.e. they can deliver andreceive fluid in two directions and therefore the pumps can be connectedin closed circuit. In this embodiment the pumps 30,31 are swash-platepumps having a swash-plate serving as a displacement volume varyingmember. The pump driving the actuator 10 associated with the squeezeplate 2 has preferably a larger capacity than the other pump, since thesqueeze plate 2 is required to move at higher speed than the pivotedsqueeze plate 3. A servo pump 35 delivers hydraulic fluid from areservoir 36 to the pumps 30,31 through a conduit 37. The pumps 30,31and 35 are coupled to a common drive shaft 33 which is driven by a motor34. Thus, the braking energy fed back to one of the pumps is transmittedto the other pump.

Each of the two ports of the first pump 30 is connected to the conduit37 via a separate conduit including a non-return valve. In an analogousmanner, each of the ports of the second pump 31 is connected to conduit37.

One of the ports of the first pump 30 is connected to the innercompartment 17 of the first linear hydraulic actuator 10. The other portis connected directly through conduit 21 to compartment 16 a and furthervia an on/off valve 38 and through a common conduit 20 to the outercompartment 16 of the first linear hydraulic actuator 10. The conduit 20is connected via an on/off valve 39 to the reservoir.

In an analogous manner, one of the ports of the second pump 31 isconnected to the inner compartment 17′ of the second linear hydraulicactuator 10′. The other port is connected directly though conduit 23 tocompartment 16 a′ and further via an on/off valve 40 and through acommon conduit 20 to the outer compartment 16′ of the second linearhydraulic actuator 10′.

The operation of the hydraulic system during the various stages of theproduction cycle of the string moulding apparatus will now be described.

For bilateral pressing the mould (FIG. 1a), valves 38 and 40 are in the“on”, i.e. the open position and valve 39 is in the “off” position. Thedirection of the pumps 30,31 is set to deliver the fluid under pressureto the ports that are connected to the conduits 21 and 23, respectively.Fluid under pressure is thus delivered to the compartments 16 a and 16a′ and through the open valves 38 and 40 to the outer compartments 16and 16′. The inner compartments 17 and 17′ are connected throughconduits 22 and 24 to the suction side of the first pump 30 and thesecond pump 31, respectively. Since the volume of compartments 17 and17′ returning fluid is smaller than that of the compartments receivingfluid, additional fluid is drawn in by the pumps 30,31 from thereservoir 36 and delivered by the servo pump 35 via the non-returnvalves. A maximum pressure on the squeeze plates 2 and 3, for pressingthe mould 5 in the chamber 1, is thus obtained.

For stripping the pivoted squeeze plate 3 from the mould 5 and forpivoting the pivoted squeeze plate 3 out of the way, the direction ofpump 31 is set to deliver fluid under pressure to the port that isconnected to conduit 24. Pressurised fluid is thus delivered to chamber17′. In order to evacuate compartment 16′, valve 39 is switched to the“on” position and the fluid is returned via the open valve 39 throughthe conduit 20 to the reservoir 36. The fluid evacuating fromcompartment 16 a′ is returned to the pump through conduit 23, since thevalve 40 is switched in the “off” position.

For pushing the mould 5 but of the moulding chamber 1 with the squeezeplate 2 (FIG. 1c), the pump 30 is set to deliver fluid under pressure tothe port that is connected to the conduit 21. Valve 38 is switched toits “off” position, thus only chamber 16 a is pressurised. The fluidevacuating from chamber 17 is returned through conduit 22 to the pump30.

For stripping-off the squeeze plate 2 from the mould 5 and for movingthe squeeze plate 2 back to its starting position (FIG. 1d), pump 30 isswitched to deliver fluid under pressure to the port connected toconduit 22. Thus, compartment 17 is pressurised. The fluid evacuatingfrom chamber 16 a is returned to the pump 30 through conduit 21, thevalve 38 is switched to the “off” position. The fluid evacuating fromthe compartment 16 is returned through conduit 20 via the open valve 39to the reservoir 36.

For returning the pivoted squeeze plate 3 to the moulding chamber 1(FIG. 1e), the pump 31 is set to deliver fluid under pressure to theport connected to conduit 23. Valve 40 is switched to its “off”position, thus only chamber 16 a′ is pressurised. The fluid evacuatingfrom chamber 17′ is returned through conduit 24 to the pump 31.

With reference to FIG. 4 the movements of the pressure plates 2 and 3are illustrated by means of a plot of the speed in m/s versus time inseconds. The line with reference numeral 50 represents the speed of thesqueeze plate 2. The line with reference numeral 52 represents the speedof the pivoted squeeze plate 3, whereas the line with reference numeral54 indicates the time in which the sand is shot into the mouldingchamber 1.

After the sand shot, the bilateral squeezing of the mould 5 is initiatedby the squeeze plate 2. The start of the pressing movement of thepivoted squeeze plate is, as explained in more detail in U.S. Pat. No.5,647,424, delayed with respect to the squeeze plate 2 in order tocompensate for the limited stroke of the pivoted squeeze plate 3. Inapparatus with an extended stroke of the pivoted squeeze plate 3, thepressing movement of the squeeze plates 2,3 can commence simultaneously.Next, the pivoted squeeze plate 3 is stripped off the mould 5 andpivoted out of the way. Before this movement of the pivoted squeeze 3plate has finished, the squeeze plate 2 starts to move further into andpast the moulding chamber 1 to push out the mould 5. This movement ishowever preferably not started before the pivoted squeeze plate 3 andits associated pattern have passed the front 1 a of the moulding chamber1. The squeeze plate 2 continues it movement to push the mould 5 beyondthe pivoted squeeze plate 3 and slows down to a complete standstill whenthe front of the mould 5 abuts with the previously produced mould 5. Themovement of the squeeze plate 2 is thereafter continued so that the lastand previously produced moulds are moved together as a stack or string 7of moulds 5. When movement of the mould string 7 is completed, themovement of the squeeze plate 2 is reversed to move back to the startingposition. Before the squeeze plate 2 has reached its starting position,the pivoted squeeze plate 3 starts to pivot and move back to themoulding chamber 1. The timing of the movement of the pivoted squeezeplate 3 back to the moulding chamber 1 is calculated taking into accountthe geometry and position versus time of the pivoted squeeze plate 3,the geometry and the position versus time of the squeeze plate 2 and theassociated patterns. Before the pivoted squeeze plate 3 has reached itsstarting position again, in which it closes the moulding chamber 1, thesand shot is started, and a new cycle begins.

According to a modified embodiment of the invention, the centring of thetwo squeeze plates is done simultaneously.

According to another modified embodiment of the invention only thesqueeze plate 2 moves during the pressing of the mould 5, whereby thepivoted squeeze plate 3 remains stationary.

According to still another modified embodiment of the invention, thepumps 30, 31 are fixed displacement pumps. In this embodiment, eitherthe speed at which the pumps are driven is varied or proportional valveare used in order to vary the amount of fluid delivered to theactuators.

REFERENCE NUMERALS  1 moulding chamber  1a moulding chamber front  2squeeze plate  3 pivoted squeeze plate  4 hopper  5 mould  6 conveyor  7mould string  8 mould-string-transporting means  9 sand injection slot10 first linear hydraulic actuator 10′ second linear hydraulic actuator11 cylinder 11′ cylinder 12 piston head 12′ piston head 12a secondpiston head 12a′ second piston head 13 piston rod 13′ piston rod 13asecond piston rod 13a′ second piston rod 14 inner end wall 14′ inner endwall 15 stationary block 16 outer annular compartment 16′ outer annularcompartment 16a compartment 16a′ compartment 17 inner annularcompartment 17′ inner annular compartment 18 outer end wall 18′ outerend wall 20 conduit 21 conduit 22 conduit 23 conduit 24 conduit 25bracket 26 push and pull rods 27 frame 28 hinge 30 first pump 31 secondpump 33 common drive shaft 34 motor 35 servo pump 36 reservoir 37conduit 38 on/off valve 39 on/off valve 40 on/off valve 50 speed ofsqueeze plate 52 speed of pivoted squeeze plate 54 sand shot

What is claimed is:
 1. Method of controlling a movement of squeezeplates of a string moulding apparatus comprising a moulding chamberlocated between a squeeze plate and a pivoted squeeze plate, at leastone of the squeeze plates being provided with a pattern, comprising thesequential steps of: a) moving the squeeze plate alone, or both thesqueeze plate and the pivoted squeeze plate, from a starting positioninto the moulding chamber to press a mould, b) moving the pivotedsqueeze plate out of the moulding chamber and pivoting the pivotedsqueeze plate to strip the pivoted squeeze plate from the mould and toopen the moulding chamber to allow the mould to be transported beyondthe pivoted squeeze plate, c) moving the squeeze plate further into andpast the moulding chamber and past the pivoted squeeze plate totransport the mould beyond the pivoted squeeze plate, d) moving thesqueeze plate back into the moulding chamber to strip the squeeze platefrom the mould and to resume the starting position thereof, e) pivotingand moving the pivoted squeeze plate back to the moulding chamber toresume the starting position thereof and to close the moulding chamber,wherein one of (i) the moving of the pivoted squeeze plate out of themoulding chamber in step b) starts before the moving of the squeezeplate to press the mould in step a) has ended, (ii) the moving of thesqueeze plate further into and past the moulding chamber in step c)starts before the moving and pivoting of the pivoted squeeze plate toopen the moulding chamber in step b) has ended, or (iii) the pivotingand moving of the pivoted squeeze plate back to the moulding chamber instep e) starts before the moving of the squeeze plate to the startingposition thereof has ended.
 2. Method according to claim 1, wherein themoving of the squeeze plate further into and past the moulding chamberin step c) starts when a mould face formed by the pivoted squeeze platereaches a mould chamber front and after when the pivoting movement ofthe pivoted squeeze plate in step b) has started.
 3. Method according toclaim 1, wherein the pivoting movement of the pivoted squeeze plate instep e) starts as soon as a collision between the pivoted squeeze plateand the squeeze plate and the associated patterns is excluded.
 4. Methodaccording to claim 1, wherein both of: (i) the moving of the pivotedsqueeze plate out of the moulding chamber in step b) starts before themoving of the squeeze plate to press the mould in step a) has ended, and(ii) the moving of the squeeze plate further into and past the mouldingchamber in step c) starts before the moving and pivoting of the pivotedsqueeze plate to open the moulding chamber in step b) has ended. 5.Method according to claim 1, wherein both of: (i) the moving of thepivoted squeeze plate out of the moulding chamber in step b) startsbefore the moving of the squeeze plate to press the mould in step a) hasended, and (iii) the pivoting and moving of the pivoted squeeze plateback to the moulding chamber in step e) starts before the moving of thesqueeze plate to the starting position thereof has ended.
 6. Methodaccording to claim 1, wherein both of: (ii) the moving of the squeezeplate further into and past the moulding chamber in step c) startsbefore the moving and pivoting of the pivoted squeeze plate to open themoulding chamber in step b) has ended, and (iii) the pivoting and movingof the pivoted squeeze plate back to the moulding chamber in step e)starts before the moving of the squeeze plate to the starting positionthereof has ended.
 7. Method according to claim 1, wherein all of: (i)the moving of the pivoted squeeze plate out of the moulding chamber instep b) starts before the moving of the squeeze plate to press the mouldin step a) has ended, (ii) the moving of the squeeze plate further intoand past the moulding chamber in step c) starts before the moving andpivoting of the pivoted squeeze plate to open the moulding chamber instep b) has ended, and (iii) the pivoting and moving of the pivotedsqueeze plate back to the moulding chamber in step e) starts before themoving of the squeeze plate to the starting position thereof has ended.